The present invention relates generally to spectroscopic interrogation of flowing gas samples without interruption of the flow, and more particularly to a purged window apparatus in which spectroscopically transparent, inert purge gases are introduced in such a manner that the window, through which probing electromagnetic radiation is inserted, and signal electromagnetic radiation is retrieved for the purpose of investigating the nature of the flowing gas, remains free of contamination by deposition or by chemical attack by substances in said flowing gas.
Monitoring of flowing systems such as coal gasifiers, for example, for contaminant gas species has become increasingly important in recent years with the continued emphasis being placed on alternative energy sources. Development of on-line, real-time gas species or particulate detection and analysis procedures has been problematic because of the destructive nature of the flowing gases. That is, mechanical or electrical probes placed in such streams are quickly destroyed by particulate caused erosion or by chemical attack by corrosive species present. Remote spectroscopic analysis has therefore emerged as a viable analytical tool for such systems. In particular, laser-induced fluorescence, coherent antistokes Raman spectroscopy, and laser-induced dielectric breakdown spectroscopy lend themselves well to such hostile environments. However, in the three above techniques laser radiation must be introduced into the flowing medium, and emitted or perhaps transmitted radiation extracted therefrom to provide diagnostic information concerning the gas composition. Moreover, other spectroscopic detection schemes might require electromagnetic radiation from sources other than lasers, so that some form of electromagnetic radiation transparent material must be present. The major difficulty with using such transparent materials is that they usually become opaque rather quickly from deposition of material in the flowing gas or from direct chemical attack, even if said transparent material is placed away from the gas flow itself; that is, offset from the area of substantial material transfer.
In the past, a common solution to this problem was to place the transmitting windows at the end of short tubes externally attached to the chamber containing the flowing gases and into which a flow of purging gas is applied over the window surface to reduce the backstreaming of the flowing gas of interest onto the windows by providing a buffer zone. However, even with a substantial flow limited only by the gas handling capability of the flow chamber, the windows deteriorate in time, reducing the sensitivity of the detection system and finally necessitating their cleaning or replacement.
The apparatus of the instant invention, through the use of tubes with longitudinally tapered interiors and a turbulent vortex flow over the window surface provides virtually complete protection from window contamination at very modest purge gas flow rates. The vortex flow is achieved by injecting the purging gas tangentially in the vicinity of the windows. This eliminates any regions of zero flow within the tubes into which flow gas can backstream. Moreover, the lower purge gas flow rates avoid dilution of the process gas under investigation, and unnecessary waste of purified gas.